The present invention relates to a surface-coated hard material, a production method for this material, and use of the same.
It is known that coats of lacquer applied to flooring laminates, genuine wood laminates, parquet, furniture, or to wood or plastic panels in general can have resin added to them in order to make them resistant to wear. When this is done, lacquer systems based on phenol, melamine, aldehyde, formaldehyde, urea, epoxy, polyester and/or polyurethane resins are used. Preferred lacquer systems are the melamine resins. Because of its hardness, transparency, inertness, and availability, aluminum oxide or alumina products in the form of fused corundum, sintered corundum, monocrystalline corundum and/or calcined or sintered alumina such as plate-like alumina are preferred for increasing the wear-resistance of the coatings.
EP 0 732 449 A1 discloses a method for producing wear-resistant laminates, in which the surface of the resin-impregnated paper that is used during the production process is coated with a mixture consisting of melamine resin, cellulose fibers, corundum as the hard material, additives and water, and is dried to a specific residual moisture content. The resin-impregnated paper is processed within the laminate in the usual way. After the laminate has been pressed and the melamine resin has hardened, the corundum is firmly bonded into the resin layer, the wear resistance of which is greatly increased because of the hardness of the corundum. DE 195 29 987 A1 discloses a method for producing highly wear-resistant lacquer coatings on a solid carrier, when wear-reducing material is either scattered directly onto the surface of the carrier and then covered with a synthetic-resin lacquer (acrylate resin, polyester resin, or polyurethane resin lacquer), or the wear-reducing agent is J scattered onto the surface of the carrier that is already coated with lacquer. Generally speaking, when this is done, the wear-reducing effect of the resin increases as the grain size increases and as the degree to which the lacquer coating is filled increases. The maximum grain size that can be used, which is, at the same time, the optimal size, is determined by the thickness of the lacquer coating. However, the optimal degree of filling does not correspond to the possible maximum but is limited by the simultaneous demand for the highest possible degree of transparency of the lacquer coating. The subsequent pressing and hardening of the lacquer is effected by using known technology. One additional variant that is available in particular for laminates is that a transparent overlay paper that incorporates the appropriate resin is impregnated with lacquer then pressed onto the decorative layer and hardened. It is preferred that corundum be used as the wear-reducing agent. Synthetic corundum is usually produced in an arc furnace, when the starting material—alumina or bauxite—is smelted at approximately 2000° C. In this process, the product is in the form of large blocks weighing several tonnes, and after cooling these are crushed and then processed into granular material. Typical areas of use for granular corundum, which is available in the most varied grades and grain sizes ranging from a few millimeters to several micrometers, are as grinding agents and refractory products. Because of its brittle-fracture behaviour, when the corundum is ground this results in a markedly fissured surface with many edge dislocations, micro-edges, grooving, and cracks. Similar grain surfaces with additional pores also seen in sintered or calcined alumina, particularly if they have been previously subjected to a grinding process. Grain surfaces of this kind display a high degree of capillarity with respect to low-viscosity liquids. Such grain characteristics have been found to be disadvantageous when processing aluminum oxide to form wear-resistant coatings.
According to the current state of the art, today, a wear-resistant lacquer coating is produced by a single application of the wear-reducing lacquer to which the hard granular material has already been mixed, subsequent drying, and pressing. When this is done, the particles of hard material lie, in part, directly on the surface of the protective coating so that, for example, dyes or other coloured liquids with a high of level of creepage penetrate irreversibly into the micro-capillaries of these hard material particles thereby causing patches that cannot be removed from the laminate or lacquer surface. Attempts that have been made to avoid this effect by using low-viscosity lacquer systems that cover the whole of the granular material surface have been unsuccessful, since a minimum degree of viscosity is needed in order to achieve the desired thickness of the lacquer coating.
A further disadvantage of the markedly fissured surfaces of the hard granular material that cannot be completely wetted with lacquer is the fact that light is scattered diffusely on the above-discussed micro-edges, cracks, and edged displacements so that the transparency of the lacquer coating that is filled with aluminum oxide is degraded. However, a high degree of transparency is one of the most important criteria for these coatings, which are frequently used in applications in which the visual effect plays a major role. In addition to this, small air bubbles can accumulate on these micro-edges and cracks, and this causes an additional defuse scattering of the light, with the result that transparency is still further degraded.
Thus, it is the objective of the present invention to describe granular hard material that does not suffer from the disadvantages described heretofore.